In many systems of treating web material it is necessary to treat the web in a continuous fashion by conveying the web through various steps. A common step is to cut the web to a desired size. Thus a slitting step is included in many operations.
In these operations, such as the treatment of polyethylene coated paper webs for inducing photographic film or paper, the slitting operation produces skivings which when settling on the web, produces contaminants which adversely affect the final product.
Thus, in the photographic field, a high quality raw stock paper is sandwiched between two layers of high grade molten polyethylene in a continuous operation at a high rate of speed.
As the polyethylene is applied to the paper, it is squeezed between cooled rollers in a pressurized nip area. As a result, some of the polyethylene is forced past the edges of the paper, creating what is known in the trade as overcoat.
This overcoat is beneficial due to the fact that it can be used somewhat to control the caliper of the papers edges, as well as lock the paper into a sealed environment with a process called edge encapsulation.
It is this overcoat material that leads to the problem of the operation.
As desirable as the overcoat is, if left on in its entirety, it causes many problems downstream of the operation, for this reason as a step in the process, it is trimmed off in a final width slitting operation.
As the paper enters the winder area, it passes through a set of rotary slitter knives which are set to a pre determined width for the particular grade being run.
These knives turn at line speed and in fact, trim off the undesirable part of the overcoat. In doing so, a small amount of polyethylene is rubbed off of the trimmed edge, and left clinging to the rotary slitter knife. As the knives collect this material (known in the business as skivings) it is gradually thrown off by centrifugal force.
As the skivings become airborne, they are picked up by the static charge created by the moving web of polyethylene coated paper and carried onto the finished product where they are crushed into the surface by the nip pressure and the mass of the finished roll at the winding operation.
As a safeguard against sparking in the sensitizing operation the paper is generally treated with an antistatic coating on one side.
As the rolls sit in storage, they lose any static charge that was built up in the winding operation and as a result, when the rolls are unwound in the next operation, some of the skivings again become airborne, and now contaminate even more of the web instead of just the edges.
Even worse, as the paper conveys through the sensitizing machine, some of the embedded particles come free and contaminate the rest of the web.
An across the web system has existed for many years, and various other systems exist to clean moving webs, but however, poly skivings are a unique problem, because they have always existed but were never diagnosed before as a contaminant. With the increasing demand on higher quality products, and the thinner more surface sensitive emulsions coming onto the market they have become a defect that has stirred up a great amount of concern in finishing. In the business of creating an absolutely flat surface, any disruption in that surface can be a major problem, especially in the photographic industry where a minute surface disruption can actually change a color, or show up as a colored spot on a finished photo. An absolutely clean base is a must.
Poly skivings are a hard defect to deal with because by nature, they are light in weight, small in size, prone to static cling and once they are in a clean environment, they are all but impossible to get rid of.
The prior art has dealt with the slitter skiving problem in many ways. In U.S. Pat. No. 2,722,983, a device is shown for cutting wire and string products. In U.S. Pat. No. 4,875,398, a device is shown which collects saw dust for slitting sheets. The removal of slitting dust is described in U.S. Pat. Nos. 3,272,651; 4,704,930; 4,300,421; 3,795,164; 3,465,625; 3,691,888; 4,003,276; and 5,480,333. Although various methods are taught to remove contaminants, the problem still remains, especially in high speed operations.